1. Field of the Invention
The present invention relates to a method for forming wirings of integrated circuits and it also relates to an integrated circuit thus formed.
2. Description of the Related Art
As the high frequency circuits are used more and more widely, they are asked to become smaller in size. Particularly in the case of semiconductor devices, an MMIC (monolithic microwave integrated circuit) in which even receiving elements are integrated is proposed.
Same thing can be said about the semiconductor module and receiving elements on a substrate are asked to become smaller in size. One of important things in making them smaller in size is to reduce loss of wirings and electrodes in a circuit.
On the other hand, the common method of making circuit elements smaller in size is to reduce the area of each element. When the circuit elements are made small in size, wiring and electrode resistances become large. Metal (Au) having a small electric resistance value is used for these wirings and electrodes. It is however difficult to make these Au-made wirings and electrodes thick by a single process. Wirings and electrodes are therefore formed thin on a wafer at a first step and metal is then laminated thick on them at a second step by electroplating. Metal having same resistance value as Au is used as plating material in this case. The term "wiring" used herein means not only a thin, wire-like conductor, but also a conductor such as a gate electrode.
Referring to FIGS. 1 through 3, the conventional method of forming wirings will be described. A substrate 1 is made of GaAs and a first insulating layer 2 made of SiO.sub.2 is formed on the substrate 1. A first wiring 3 is formed on the first insulating layer 2. A second wiring 5 and a second insulating layer 4 made of SiO.sub.2 are further formed on them.
In order to form a third wiring 9 at the electroplating process which will be later carried out, a thin under metal layer (or metal layer through which plating current is supplied) 6 is then formed all over the wafer to thereby also electrically connect all of the wirings 5. The under metal layer 6 serves as a plating metal depositing electrode at the electroplating process. Metal which allows plating metal to be easily deposited on it is therefore used to form the under metal layer 6. Further, metal which is same in kind as the plating metal is usually used to form the under metal layer 6, because it can provide excellent close contact and smooth surface. When metal which is quite different in composition from the plating metal is used to form the under metal layer 6, the plating metal 9 is likely to separate and peel off from the under metal layer 6. Further, Au particles electroplated have a diameter larger than usual ones and the layer plated by them has a rough surface. Wirings or electrodes thus formed have a high resistance.
Conventionally, Au is often used as wirings of the electric circuit and it is also used as the under metal. In order to conduct the plating selectively, a mask 7 is formed, as shown in FIG. 1, on that area of the wafer where no plating is conducted. The mask 7 is an insulating material such as photo resist.
The electroplating is then conducted, as shown in FIG. 1, while using the under metal layer (Au) 6 as a cathode. As shown in FIG. 2, the plated metal layer 9 is formed to have a thickness larger enough than that of the under metal layer 6. The mask 7 is all removed after the electroplating and the under metal layer 6 is also removed as shown in FIG. 3.
However, Au is one of the most difficult metals to remove. Two methods are conventionally used to remove the under metal layer 6 which is made of Au. One of them is to etch all over the under metal layer 6 by ion milling. The other is to etch all over the under metal layer 6 by a composition of the cyanic group. Whichever may be used, matters (wirings 5, 9 and under metal layer 6) exposed on the surface of the wafer are made of Au. A part of them is therefore removed. In short, it is allowed in conventional cases that they are a little etched, as shown in FIG. 3. When the etching condition changes in removing the under metal layer 6, therefore, they are sometimes over-etched. In the worst case, they vanish. Particularly when the wiring has a narrow line width about two times that of the under metal layer 6, it is strongly affected by the over-etching. Further, manufacturing cost is increased when the ion milling method is used because the apparatus needed by the method is high in cost.
On the other hand, cyanogen must be used in plating solution at the electroplating process. Cyanogen which is a poisonous matter becomes a problem for reasons of safety and sanitation when it is disposed as a waste.